1. Field of the Invention
The present invention relates to a method of manufacturing a stamper for use with an optical information recording medium, and in particular, to a method of forming a a pattern in the stamper.
2. Discussion of the Background Art
Regarding a general photoresist, since the latent image is formed by the action of the bridge reaction and the thermal bridge reaction due to the exposure, the width of the groove at the opening portion (the length of the long side in the trapezoidal shape: W.sub.top) turns out to be wider by approximately 10%-20% than the diameter of the beam spot. Furthermore, since the light intensity distribution of the focused beam is a Gaussian distribution, the groove formed on the photoresist becomes trapezoidal in shape. Refer to FIG. 6 and FIGS. 8-10.
The problem with a trapezoidal groove is that, when the track pitch is narrowed, the opening portion of the groove mutually interferes with the tracks adjacent thereto, and the height of the flat portion (land) between the grooves decreases, and thereby the depth of the groove cannot be controlled by the thickness of the photoresist film. Another problem is that the recording property of an optical information recording medium made by a stamper not having flat lands is deteriorated. The cross-talk signal from the adjacent tracks increases, in particular, and the jitter property is worsened.
For this reason, in a stamper for use with an optical information recording medium having narrow track pitch and large capacity, the width of the groove should be narrow and the shape of the groove cross section should be substantially rectangular. In order to narrow the groove to be formed on the photoresist, the wavelength of the exposing beam is preferably shortened and the numerical aperture (referred to hereinafter as "NA") of the opening portions of the object lens is preferably increased. However, since the rate of the focal point depth is decreased, there arises a fear that the shape of the groove varies. Therefore, it is necessary to realize a photolithograph technology in which a groove cross section finer than the exposing beam spot can be obtained even though the wavelength is not decreased and the NA is not increased.
Regarding the background art for forming a groove having a diameter not larger than the exposing beam spot, the following three methods have been adopted:
(1) a method of reinforcing (increasing) the contrast by use of a light decoloring resin;
(2) a method of decreasing the diameter of the beam spot by use of an ultra-resolution element; and
(3) a method of etching the lower layer utilizing a resist mask pattern.
As a representative example of the aforementioned item (1), the published specification of Japanese Laid-open Patent Publication No. 7-287,874/1995 describes a method of manufacturing the master plate of an optical disc. In the technology, a light decoloring resin layer is formed on the photoresist, and the diameter of the practicing beam spot to be focused on the photolithographic resist by the photochromic effect can be decreased to the extent of 2/3-1/2.
As a representative example of the aforementioned item (2), the published specification of Japanese Laid-open Patent Publication No. 6-243, 511/1994 describes the method of making the master plate for use in the optical disc. In the technology, an ultra-resolution element is inserted into the optical path (light path), and thereby the diameter of the beam spot is reduced. Since the phenomenon of the sidelobe occurs due to the influence of the diffraction of the ultra-resolution element, the combination of the ultra-resolution element and the light decoloring resin is employed for coping with the above-mentioned phenomenon.
As a representative example of the aforementioned item (3), the published specification of Japanese Laid-open Patent Publication No. 9-106,584/1997 describes the method of manufacturing a recording master plate for use with an optical disc. In the technology, utilizing the fact that the shape of the cross section of the pattern to be formed on the photoresist, an intermediate layer formed under the resist is etched with the resist pattern serving as the mask. Furthermore, the lower layer is then etched with the intermediate layer serving as the mask. In such manner, it turns out to be possible to form a groove finer than the diameter of the beam spot (groove pattern of the resist) having the cross section almost vertical.
Regarding the background art as described in the item (1), the diameter of the beam spot focused on the surface of the photoresist can be reduced to almost 50% owing to the contrast reinforcing effect due to the light decoloring resin layer. However, the amount (intensity) of the light arriving at (transmitted through) the photoresist surface is lowered to approximately 1/10. This is a problem of the background art to be solved. Refer to items [0025] and [0027] on page 3 in the published specification of Japanese Laid-open Patent Publication No. 7-287,874/1995.
Here, in order to increase the light intensity of the exposing beam, it is necessary to prepare a large-size laser light source of high power. As the result, the operating cost of the laser increases. In addition, it is necessary to further increase the light intensity in order to raise the exposure straight line velocity (CLV) for shortening the time of exposing the master plate. On this occasion, there arises a problem of the shortage of the light intensity. That results in the limitation of the improvement in productivity.
Regarding the background art as described in item (2), the diameter of the exposing light beam is reduced by inserting the ultra-resolution element into the optical path. However, even in the ultra-resolution element, the light intensity is reduced to almost 1/2 due to the light loss. Refer to item (0018) on page 4 in the published specification of Japanese Laid-open Patent Publication No. 6-243,512/1994. In order to remove the influence of the sidelobe due to the ultra-resolution element, it is necessary to reinforce the contrast due to the light decoloring resin layer. This causes a further shortage of light intensity as compared with the above background art (1).
Regarding the background art as described in item (3), in the background method utilizing the photoresist layer or the intermediate layer as the mask (the method described in the published specification of Japanese Laid-open Patent Publication No. 106,584), the master plate is constructed with the three layers of etching material (lower layer)/Sio.sub.2 (Intermediate layer)/photoresist (upper layer). The lower layer is etched with the intermediate layer as the mask layer and the groove pattern on the lower layer thus etched (etching layer) is used as d stamper.
In the method of forming the groove by use of a single-layer photoresist film, two more processes are added: a process of forming the films on the Intermediate layer and the lower layer and a process of etching the intermediate layer and the lower layer, the manufacturing yield for the product (stamper) is lowered due to the increase of the defects occurring in those processes. Furthermore, the cost of manufacturing the stamper is increased due to the increase of the processes thereof. Those are problems to be solved.
Furthermore, in the case of making the stamper by processing the lower layer surface having a groove formed thereon with the conductive film treatment and Ni-electroforming, it is necessary to remove the etching layer remaining on and attached to the surface of the stamper. In order to remove it, the same method as that of the method of etching the lower layer thereof is utilized. However, on this occasion, the surface of the stamper is damaged thereby and the noise level at the time of recording/reproducing increases. Those are problems to be solved.